I tried to don't load them in boot time, I commented the two lines then reboot. I tried to load them after with the kldload command with the path from root as you did but the message is exactly the same.
In case you ask, of course I uninstalled the package nvidia-driver from the repositories. I suppose it won't let me install a built package if it is already installed, eventually build and make the package but it will stop at the moment when it tries to install it so yes, I uninstalled it before to run "make -C /usr/ports/x11/nvidia-driver install clean clean-depends".

Ok I found the solution to my problem, I simply built an up to date FreeBSD kernel from sources. I don't know how to update the kernel the normal way, I got used to do it like that with NetBSD and it worked so I guess it's cool.
Oh and I was able to uncomment the two nvidia_load from loader.conf, they work now.

So when on FreeBSD, to update the kernel, you can only fetch it already built or build it by yourself as on other BSD instead of using a "pkg" like command to fetch it from repositories as on GNU/Linux, right ?